Although saturated aldehydes can be relatively easily oxidized to carboxylic acids, it is generally recognized that the oxidation of an unsaturated aldehyde to the corresponding carboxylic acid is accompanied by undesirable side reactions that reduce yield. In particular, it has been difficult to prepare methacrylic acid from methacrolein, an unsaturated aldehyde, in good yield. Whereas a saturated aliphatic aldehyde can generally be converted without difficulty to the corresponding carboxylic acid in high yield, this does not apply to unsaturated aldehydes. One of several problems encountered with the oxidation of unsaturated aldehydes to the corresponding acids is polymerization through the olefinic unsaturation and another problem is that of side reactions or of degradation of the materials present during the reaction with the result that less desirable byproducts are formed. The use of conventional polymerization inhibitors is a possible method of restricting such polymerization; but generally these inhibitors are also well known antioxidants and therefore also inhibit the oxidation reaction.
It is known that an unsaturated aliphatic aldehyde can be oxidized to the corresponding acid in liquid phase by means of an oxygen containing gas, such as air, being introduced into the reaction medium or mixture. Generally the oxidation is carried out in the presence of certain heavy metals such as inorganic or organic salts of cobalt, copper, nickel, manganese, silver, vanadium, iron, chromium, etc., that function as oxidation catalysts.
Although liquid phase oxidation of unsaturated aldehydes to the corresponding acids is generally conducted with a metal catalyst, a method is described in U.S. Pat. No. 3,114,769 wherein methacrolein is oxidized to methacrylic acid in the presence of a small quantity of iodine without the addition of a metal catayst. However the liquid phase oxidation of unsaturated aldehydes to the corresponding acids, such as methacrolein to methacrylic acid, has generally been carried out with a metal oxidation catalyst in various inert solvents such as hydrocarbons, chlorinated hydrocarbons, amines, esters, etc. U.S. Pat. Nos. 2,153,406 and 3,155,719 are representative teachings that suggest the use of such solvents including chlorinated solvents such as carbon tetrachloride, chloroform (trichloromethane), ethylene dichloride, and chlorobenzene.